Taking clear and sharp photographs generally requires focusing the lens of a camera on the object being photographed. Traditionally, the photographer manually focuses the camera lens for the photograph being taken, so that an image of the object lies on the plane of the film or image sensor that captures the image. This manual focusing can take some time and skill. Accordingly, cameras with autofocusing have been developed to simplify photography.
FIG. 1 schematically illustrates a digital camera 100 containing a known autofocus system. Camera 100 includes a lens 110 that is mounted on a drive mechanism 115 capable of changing the distance between lens 110 and an image sensor 120, and a motor 130 under the control of a controller 140 moves lens 110 for focusing on a target object. A pulse generator 135 measures the rotation of motor 130 and generates a pulse signal that controller 140 can use for tracking movement of lens 110. For autofocusing, a range finder 150 that receives light through lens 110 determines a distance between an image plane of lens 110 and the plane of image sensor 120. Range finding may, for example, be implemented using known phase difference detection techniques. Controller 140 receives the distance determined in range finder 150 and activates motor 130 to move lens 110 the distance required to place the image plane of lens 110 on image sensor 120. When lens 110 is properly focused and the photographer operates the shutter button of camera 100, image sensor 120 captures the image, which can be digitally stored in a memory 160.
The autofocus system described above generally adds to the cost of camera 100. In particular, camera 100 requires specific components such as range finder 150 with associated optics (not shown) and pulse generator 135 that may only be used for autofocusing. Additionally, motor 130 and drive mechanism 115 must be able to move lens 110 a controlled distance in either direction, and a relatively expensive stepper motor or voice coil motor may be required for adequate control of drive mechanism 115.
An autofocus system is desired that can reduce camera cost and/or size through reduction of the number and cost of autofocusing components.